Temperature monitoring

ABSTRACT

This application relates to methods and apparatus for temperature monitoring for integrated circuits, and in particular to temperature monitoring using a locked-loop circuits, e.g. FLLs, PLLs or DLLs. According to embodiments a locked-loop circuit ( 200, 600 ) includes a controlled signal timing module ( 201, 601 ), wherein the timing properties of an output signal (S OUT , S FB ) are dependent on a value of a control signal and on temperature. A controller ( 201, 601 ) compares a feedback signal (S FB ) output from the timing module to a reference signal (S REF ) and generates a control signal (S C ) to maintain a desired timing relationship. A temperature monitor ( 202 ) monitors temperature based on the value of the control signal. For FLLs and PLLs the signal timing module may be a controlled oscillator ( 201 ).

TECHNICAL FIELD

This application relates to methods and apparatus for temperaturemonitoring, in particular for temperature monitoring of integratedcircuits and to detecting thermal runaway.

BACKGROUND

Temperature monitoring of circuitry may be useful for a variety ofreasons. For example it may be desirable in some instances to monitorwhether the temperature of the circuitry exceeds a defined operatingrange, which could be a safe operating range. Integrated circuits willgenerally generate heat in use. Elevated temperature can increaseleakage currents, which may lead to more heat generation and furtherincrease in temperature, which can lead to thermal runaway which couldresult in malfunction and/or damage of the circuitry and possibly thehost device.

With a trend towards ever smaller process node geometries for integratedcircuits, the issue of heat generation is becoming more important. Suchintegrated circuits may have a greater density of active componentsleading to a greater power dissipation per unit area of semiconductor.Smaller process node geometries may have more issues with heatgeneration, for example small geometry ICs may experience greaterleakage, e.g. gate leakage currents due to thinner gate dielectrics orgreater sub-threshold drain-source leakage currents at the loweroperating voltages etc., which leakage currents may inherently increaserapidly with temperature.

It is known therefore that dedicated sensors may be provided as part ofan integrated circuit for monitoring aspects of the circuit operation inuse, including temperature. For instance PVT (process, voltage andtemperature) sensors are known for use on ICs.

Such PVT monitors can usefully monitor temperature at a location of acircuit, but do require some chip area to implement the monitor, thusadding to the size and cost of the circuitry. Such sensors may alsoinvolve analogue circuitry requiring at least some analogue circuitdesign and, for at least some applications, there is a desire tominimise analogue circuitry. Also such temperature sensors may requireinitial calibration to operate correctly which may add to thetesting/calibration steps required for the circuitry.

SUMMARY

Embodiments of the present disclosure relate to advantageous methods andapparatus for temperature monitoring.

Thus according to an aspect there is provided a locked-loop circuitcomprising:

-   -   a controlled oscillator operable to generate an output signal at        an output frequency based on the value of a control signal; and    -   a controller operable to compare a feedback signal derived from        the output signal to a reference signal received at a reference        signal input to generate said control signal;    -   wherein the controlled oscillator is operable in a first mode in        which a value of the control signal required to maintain a        certain output frequency changes with temperature; and    -   the circuit comprises a temperature monitor for monitoring        temperature based on the value of the control signal.

The temperature monitor may be configured to monitor the value of thecontrol signal against one or more thresholds and to generate an alertif said thresholds are crossed. In some implementations the circuit maybe operable such that output frequency is variable in use. In such acase the one or more thresholds may be selected based on an indicationof the output frequency. The temperature monitor may be configured tomonitor temperature based the value of the control signal and anindication of the output frequency.

In some instances the temperature monitor may be configured to determinean estimate of present temperature based on the value of the controlsignal. If the circuit is configured such that output frequency isvariable in use, the temperature monitor may be configured to determinean estimate of present temperature based on the value of the controlsignal and on an indication of the output frequency.

The temperature monitor may configured to receive a version of theoutput signal and determine the indication of the output frequency. Insome instances the locked-loop circuit may comprise a frequency dividerconfigured to apply frequency division to a version of the output signalbased on a controllably variable division value to provide the feedbacksignal. The temperature monitor may be provided with the division valueas, at least part of, the indication of the output frequency.Additionally or alternatively the temperature monitor may be configuredto receive an indication of the reference frequency as, at least partof, said indication of the output frequency.

In some implementations the temperature monitor may be configured tomonitor the value of control signal for changes that indicate acontinuing increase in temperature.

The controlled oscillator may comprise a bias circuit for generating abias for the controlled oscillator. The bias circuit may be configuredto generate the bias having a controlled temperature dependence so as toprovide a first transfer function for the controlled oscillator betweenoutput frequency and value of the control signal, wherein the firsttransfer function has a distinct temperature dependence. The controlledoscillator may comprise a current controlled oscillator and aprogrammable current source controlled by the control signal. The biascircuit may supply the bias to the programmable current source.

In some implementations the control signal comprises a digital signalhaving a value defined by a digital value.

In some implementations, in the first mode, the controlled oscillatorhas a first transfer function between output frequency and value of thecontrol signal, and the controlled oscillator is further operable in asecond mode with a second transfer function between output frequency andvalue of the control signal that exhibits a lesser temperaturedependence than the first transfer function. The second transferfunction may exhibit substantially no variation with temperature overthe defined temperature range. The first transfer function may exhibit avariation with temperature over the defined temperature range which hasa magnitude which is of the order of 15% or greater, or 20% or greater,or 25% or greater than that for the second transfer function. Thecircuit may be configured to be responsive to a control signal to enableor disable temperature sensing. The controlled oscillator may becontroller to operate in the first mode when temperature sensing isenabled and in the second mode when temperature sensing is not enabled.In some implementations the bias circuit discussed earlier may bepresent and may be configurable so as to provide a bias with a firsttemperature dependence in the first mode and a second temperaturedependence in the second mode.

In some embodiments a switching arrangement may be located in a signalpath between the controller and the controlled oscillator, and thecircuit may be operable in a closed-loop mode in which the switcharrangement is configured to supply the control signal from thecontroller to the controlled oscillator. The circuit may also beoperable in an open-loop mode, in which the switch arrangement isconfigured to supply a defined control value to the controlledoscillator as the control signal. In the open-loop mode the temperaturemonitor may be configured to receive a version of the output signalgenerated in response to the defined control value and to monitortemperature by monitoring the frequency of the output signal.

The locked-loop circuit may any type of locked-loop circuit, and in someimplementations may be a frequency-locked-loop circuit or aphase-locked-loop circuit.

The locked-loop circuit may be implemented as an integrated circuit.

Embodiments also relate to an electronic device comprising a locked-loopcircuit as described in any of the variants above. The device may be atleast one of: a portable device; a battery powered device; acommunications device; a mobile or cellular telephone; a smartphone; acomputing device; a notebook, laptop or tablet computing device; awearable device; a smartwatch; a voice-controlled device; a gamingdevice.

In another aspect there is provided a method of temperature monitoringcomprising:

-   -   operating a locked-loop circuit to compare a feedback signal        derived from an output signal to a reference signal received at        a reference signal input to generate a control signal; and    -   monitoring temperature based on the value of the control signal.

In a further aspect there is provided a locked-loop circuit comprising:

-   -   a controlled signal timing module operable to generate an output        signal; and    -   a controller operable to compare a feedback signal, output from        the controlled signal timing module, to a reference signal        received at a reference signal input to generate a control        signal for controlling the controlled signal timing module;    -   wherein the controlled signal timing module is operable in a        first mode in which a value of the control signal required to        maintain a certain timing relationship between the feedback        signal and the reference signal changes with temperature; and    -   the circuit comprises a temperature monitor for monitoring        temperature based on the value of the control signal.

In a further aspect there is provided a locked-loop circuit comprising:

-   -   a reference signal input for receiving a reference signal;    -   a controlled signal timing module operable in a first mode to        output a feedback signal, wherein the timing properties of the        feedback signal are dependent on a value of a control signal and        on temperature;    -   a controller operable to compare the feedback signal to the        reference signal and thus to generate the control signal at that        control signal value required to maintain a certain timing        relationship between the output signal and the reference signal        at the current temperature; and    -   a temperature monitor for monitoring temperature based on the        value of the control signal.

The locked-loop circuit of these further aspects may be any of afrequency-locked-loop or a phase-locked loop, in which case the signaltiming module may comprise a controlled oscillator and embodiments maybe implemented according to any of the variants discussed above. Thelocked-loop circuit could further be a delay-locked-circuit, in whichcase the signal timing module may comprise a delay line. The variantdiscussed above relating to the configuration and operation of thetemperature monitor apply equally to a delay-locked-circuit and thedelay line may be operable in various modes as also discussed above.

In a yet further aspect there is provided a locked-loop circuitcomprising:

-   -   a controlled oscillator operable in a first mode with a first        transfer function that varies with temperature such that a value        of a control signal required to maintain a certain output        frequency changes with temperature across a defined temperature        range and further operable in second mode in with a second        transfer function that exhibits a lesser temperature dependence        than the first transfer function; and    -   a temperature monitor for monitoring temperature based on the        relationship between the value of the control signal and the        output frequency when the controlled oscillator is operating in        said first mode.

In a yet further aspect there is provided a temperature monitoringcircuit comprising:

-   -   a controlled oscillator operable with a first transfer function        between a value of an input control signal and an output        frequency that varies with temperature; and    -   a temperature monitor for monitoring temperature based on the        relationship between the value of the control signal and the        output frequency;    -   wherein the controlled oscillator forms part of a locked-loop        circuit.

In another aspect there is provided a locked-loop circuit comprising:

-   -   a controlled delay line operable to receive an input signal,        apply a controlled delay to the input signal and output at least        a first signal based on the delayed input signal wherein the        controlled delay applied is based on the value of a control        signal; and    -   a controller operable to compare a feedback signal derived from        the first signal to a reference signal received at a reference        signal input to generate said control signal;    -   wherein the controlled delay line is operable such that a value        of the control signal required to maintain a certain delay        changes with temperature; and    -   the circuit comprises a temperature monitor for monitoring        temperature based on the value of the control signal.

In another aspect there is provided a locked-loop circuit comprising:

-   -   a reference input for receiving a reference signal;    -   a controlled delay line operable to apply delays to the        reference signal to provide one or more delayed signals and to        output a feedback signal based on one or more of the delayed        signals, wherein the delays are controlled dependent on a value        of a control signal, wherein the controlled delay line is        operable in a mode in which the delays are also dependent on        circuit temperature; and    -   a controller operable to generate said control signal at a        control signal value required to maintain a timing relationship        between the feedback signal and the reference signal at the        current temperature;    -   wherein the circuit comprises a temperature monitor for        monitoring temperature based on the value of the control signal.

The various features and variants described with respect to any aspectabove may also be used with or applied to the other aspects, unlessexpressly indicated otherwise and the various features may beimplemented in any combination, unless clearly indicated as beingincompatible.

BRIEF DESCRIPTION OF THE DRAWINGS

To better explain and illustrate aspects of the present disclosure,various embodiments will now be described, purely by way of exampleonly, with reference to the accompanying drawings, of which:

FIG. 1 illustrates an example of a locked-loop circuit;

FIG. 2 illustrates an example of a locked-loop circuit operable toprovide temperature monitoring;

FIGS. 3a and 3b illustrate examples of transfer functions of acontrolled oscillator with different temperature dependence;

FIG. 4 illustrates a controlled oscillator arrangement;

FIG. 5 illustrates another example of a locked-loop circuit operable toprovide temperature monitoring;

FIG. 6 illustrates a delay-locked loop circuit operable to providetemperature monitoring; and

FIG. 7 illustrates one example of a delay line.

DETAILED DESCRIPTION

Embodiments of the present disclosure relate to temperature monitoringand in particular to the use of a controlled signal timing module, suchas a controlled oscillator, for temperature monitoring, where thecontrolled signal timing module, e.g. oscillator, is also operable as apart of a locked-loop circuit for generating a desired output signalwith a locked timing relationship to the input signal. In at least someembodiments a controlled oscillator may thus be operable as part of alocked-loop circuit, for example as part of a frequency-locked-loop forfrequency synthesis, but is also operable to provide temperaturesensing.

Embodiments thus relate to temperature monitoring using components ofthe circuit that are not just provided for monitoring the operation ofthe circuit and instead may be able to provide some other functionality.The temperature monitoring may therefore be largely provided bycomponents that would, in any case, be included in the circuit for otherpurposes. Embodiments may thus effectively utilize circuit componentsthat may already be present in a circuit design to provide temperaturemonitoring. This can make it economic in terms of chip area to providethe ability for additional temperature monitoring capability to thatprovided from a dedicated PVT monitor, for instance for distributedtemperature sensing at different locations of an integrated circuit, orin some embodiments may allow for temperature monitoring of a circuitwithout requiring a dedicated temperature sensor to be provided.

Locked-loop circuits, for example frequency-locked loops (FLLs),phase-locked-loops (PLLs) or delay-locked-loops (DLLs), are known andused in a variety of applications. Locked loop circuits typicallycomprise a signal timing module that provides an output signal that hasa controlled timing relationship, i.e. a controlled relationship offrequency and/or phase, to an input signal. The input signal is anoscillating signal, such as a clock signal and may be a referencesignal. A feedback signal derived from the signal timing module, whichin some instances may be a signal tapped from the output signal or whichmay be a separate signal output from the signal timing module, is, via afeedback path, compared to the input signal to control the signal timingmodule to maintain a desired locked relationship between the input andfeedback signal, and hence the input and output signals. For FLLs andPLLs the signal timing module may comprise a controlled oscillator thatgenerates an oscillation signal that is output as the output signal. Thecontrolled oscillator may, for example comprise a ring oscillator formedfrom a ring of delay elements. For DLLs the signal timing module maycomprise a delay line, e.g. comprising a series of delay elements.

FIG. 1 illustrates the basic principle of a FLL 100 for frequencysynthesis. A controlled oscillator 101 is controlled by a controller 102to provide an output signal S_(OUT) having a desired output frequencyF_(OUT). The controller 102 thus provides a control signal S_(C) to thecontrolled oscillator 101 so as to cause it to generate the outputsignal S_(OUT). The output frequency F_(OUT) will vary with the controlsignal S_(C) according to a transfer function K_(OSC) for theoscillator, such that F_(OUT)=K_(OSC)(S_(C)). The control signal mayphysically be a voltage value or current value or may be a digitalcontrol value, i.e. a control word.

To stabilise the output signal S_(OUT) at the desired output frequencyF_(OUT), the controller 102 compares a feedback signal S_(FB), which isderived from the output signal S_(OUT), to a reference signal S_(REF) ata defined reference frequency F_(REF). Typically the controller 102 hasan error block 103 that determines the extent of any frequency error ΔFbetween the frequency F_(FB) of the feedback signal S_(FB) and thereference frequency F_(REF). The controller may also have an integrator104, or some other loop filter, which generates a control signal S_(C)for the controlled oscillator 101. A frequency divider 105 may bearranged in the feedback path so that the feedback signal S_(FB)provided to the controller 102 has a frequency F_(FB) which is dividedby a certain value N compared to the frequency F_(OUT) of the outputsignal S_(OUT), i.e. F_(FB)=F_(OUT)/N.

The operation of the FLL 100 is such that the controller 102 adjusts thecontrol signal S_(C) so as to control the oscillator 101 to reduce thefrequency error ΔF to zero. This occurs when the frequency F_(FB) of thefeedback signal S_(FB) is equal to the reference frequency F_(REF). Thusthe output frequency F_(OUT) is stabilised to a value of N·F_(REF). Thefrequency divider 105 may be controllable to apply different divisionvalues, i.e. different values of N, so that the output frequency F_(OUT)can be controlled to a desired multiple of the reference frequencyF_(REF). In some embodiments the value of N may be rapidly modulated inuse, to define a non-integer average value of N and thus providenon-integer scaling of frequency.

It will be understood that the output frequency F_(OUT) is thusstabilised to a desired multiple of the reference frequency F_(REF). Thestability of the reference frequency F_(REF) thus clearly has an impacton the stability of the output frequency F_(OUT). The reference signalS_(REF) thus should preferably have a stable frequency, i.e. a frequencystable to temperature and supply variations etc. In some implementationsa suitably accurate reference signal may be supplied from a sourceexternal to the integrated circuit comprising the FLL 100 and may bereceived at some suitable contact, e.g. a pad or pin of the circuit.Alternatively the reference signal S_(REF) may be generated on part ofthe same integrated circuit as the FLL 100 using a suitablytemperature-insensitive oscillator arrangement as would be readilyunderstood by one skilled in the art. One external source or on-chiposcillator arrangement may be used to generate a single signal S_(REF)having a stable reference frequency F_(REF) which can then be suppliedto each of a plurality of FLLs (and/or other locked-loop circuits) togenerate respective suitable output signals S_(OUT) at different desiredreference frequencies.

The transfer function K_(OSC) of the controlled oscillator 101 may havea temperature dependence. In some locked-loop designs the controlledoscillator 101 may be arranged to have a transfer function that isrelatively flat with temperature, i.e. K_(OSC)(T)≈constant. However,because the output frequency F_(OUT) is stabilised by the action of thefeedback loop, temperature stability of the transfer function K_(OSC) ofthe controlled oscillator 101 is not required to maintain atemperature-stable output frequency F_(OUT).

Some embodiments of the present disclosure thus utilise a controlledoscillator, which is arranged as a signal timing module of part of alocked-loop circuit, where the controlled oscillator is operable with atransfer function K_(OSC) that varies with temperature. If, thecontrolled oscillator operates with a transfer function K_(OSC) thatvaries with temperature, and the variation of the transfer functionK_(OSC) with temperature is predictable, and of sufficient degree suchthat the value S_(C)(F_(OUT)) of the control signal S_(C) required tomaintain a certain output frequency F_(OUT) will change significantlywith temperature, then it is possible to determine information about theoperating temperature based on an indication of the value of the controlsignal S_(C)(F_(OUT)). In some instances, if the variation of transferfunction with temperature is characterised, at least to some degree,then it may be able to determine an estimate of temperature (of thecontrolled oscillator). In some instances however monitoring the valueof the control signal S_(C) for changes may indicate temperaturevariations such as associated with thermal runaway.

As noted above the output frequency F_(OUT) depends on the value of thecontrol signal S_(C) and the transfer function K_(OSC), such thatF_(OUT)=K_(OSC)(S_(C)). If the transfer function varies with temperaturesuch that the transfer function K_(OSC)(T₁) at a first temperature T₁ isdifferent to the transfer function K_(OSC)(T₂) at a first temperatureT₂, then a different value of control signal S_(C) will be required atthe first temperature as compared to the second temperature to maintaina desired output frequency.

FIG. 2 illustrates one embodiment of a circuit 200 operable as alocked-loop circuit but with temperature monitoring capability. Similarcomponents to those illustrated in FIG. 1 are identified using the samereference numerals.

The circuit 200 of FIG. 2, like the FLL illustrated in FIG. 1, has acontroller 102 and a feedback loop which may include a frequency divider105. The embodiment of FIG. 2 however includes a controlled oscillator201 which is operable with a transfer function K_(OSC) that varies withtemperature in a defined way. In use as an FLL the circuit 200 mayoperate as described above.

The circuit 200 also includes a temperature monitor block 202 whichreceives a version of the control signal S_(C). By monitoring the valueof the control signal S_(C) the temperature monitor 202 can effectivelymonitor the temperature.

In some embodiments, based on the value of the control signal S_(C) andknowledge of or an indication of the output frequency F_(OUT), thetemperature monitor block 202 may determine an estimate of temperatureT. As described above the value of the control signal S_(C) and theoutput frequency F_(OUT) are related by a temperature-dependent transferfunction K_(OSC) such that F_(OUT)=K_(OSC)(S_(C)) where the transferfunction K_(OSC) varies with temperature. From this relationship, if thevalue of F_(OUT) is effectively known then the value of S_(C) requiredto generate this given output frequency F_(OUT) for a given temperaturemay be derived. More generally, the value of S_(C) required for each ofa set of temperatures may be derived and used to construct a transferfunction of S_(C) versus temperature for this value of F_(OUT), andinversely, i.e. a transfer function T_(S) between temperature T andcontrol signal S_(C) wherein T=T_(S)(S_(C)). The transfer function maybe constructed by a curve-fitting process, for example a polynomialcurve fitting process. Thus based on the present value of the controlsignal S_(C) an estimate of the present temperature of the controlledoscillator can be determined, which will be an indication of the ambienttemperature of the circuit at that location.

In some embodiments the output frequency F_(OUT) of the FLL circuit 200may effectively be predetermined as limited to a single value. Forinstance if the FLL 200 is designed to operate with a defined referencesignal S_(REF) at a constant, known, reference frequency F_(REF) and thedivision value N of the frequency divider 105 (if present) is alsoconstant, then the value of the output frequency F_(OUT) is effectivelyfixed and known in advance. In which case the temperature monitor 202may simply receive the value of the control signal S_(C) required toprovide this output frequency F_(OUT) and determine an indication oftemperature therefrom based on a single transfer function T=T_(S)(S_(C))derived with respect to the single value of output frequency F_(OUT).

In some embodiments the output frequency F_(OUT) may be variable in use,for instance by varying the division value N of the frequency divider105. In such a case the temperature monitor 202 may also receive anindication of the division value N. If the reference frequency F_(REF)is fixed, the division value defines the output frequency F_(OUT)according to F_(OUT)=N·F_(REF) as discussed above.

Additionally or alternatively in some instances the reference frequencyF_(REF) of the reference signal S_(REF) may itself be variable. In whichcase the temperature monitor 202 may be provided with an indication ofthe reference frequency F_(REF). This could for instance be anindication of a system operating mode, where the reference frequency foreach of a plurality of operating modes is predetermined and known oravailable to the temperature monitor 201.

In some embodiments the reference frequency F_(REF) or the outputfrequency F_(OUT) itself could be determined by some frequencymonitoring component to provide an indication of the indication of theoutput frequency, either directly if the output frequency F_(OUT) itselfis measured or as N·F_(REF) is the reference frequency F_(REF) ismeasured.

In any case, in the event that the output frequency F_(REF) is variablethen the temperature monitor 202 may, in some embodiments, be providedwith sufficient information to identify an operating regime of the FLLcircuit in terms of the expected or measured output frequency F_(OUT).For instance, the information may comprise a separately defined transferfunction T_(S)(S_(C)) for each anticipated value of the output frequencyF_(OUT).

In some implementations the temperature monitor 202 may determine anestimate of temperature based on the value of the control signal S_(C)and the operating regime, i.e. the indication of the expected ormeasured output frequency F_(OUT). In such a case the value of controlsignal S_(C) required to generate the particular output frequencyF_(OUT) at a range of temperatures may have been characterised inadvance by simulation or by measurements and used to derive a functionT_(S)(S_(C)) which can be used to determine an estimate of temperaturefrom the value of control signal S_(C).

In embodiments where the locked-loop circuit may operate in differentoperating regimes wherein the desired output frequency F_(OUT) may takedifferent respective values, the temperature monitor may receive anindication of the current desired value of F_(OUT). This may for exampletake the form of a digital word that directly represents the value ofF_(OUT). Alternatively the indication may take the form of digital wordsthat represent the current values of reference frequency F_(REF) and/ordivision value N. The temperature monitor may thus (if appropriate)determine the operating regime, e.g. based on an indication of desiredoutput frequency F_(OUT), reference frequency F_(REF) and/or divisionvalue N, and then use the present value of control signal S_(C) and anappropriately selected transfer function T_(S)(S_(C)) to determine anestimate of temperature T. In some embodiments a plurality of discretetransfer functions T_(S)(S_(C)) may be stored, each corresponding toparticular values of say F_(OUT). In some embodiments a functionT_(C)(S_(C), F_(OUT)) may be stored describing a surface in a space withcoordinates (S_(C), F_(OUT) and T), effectively interpolating betweenthe discrete transfer functions to provide coverage for a continuousrange of F_(OUT). The interpolation may be performed using known surfacefitting algorithms, for example a least-mean-squares multi-dimensionalpolynomial algorithm.

The digital words indicative of desired output frequency F_(OUT),reference frequency F_(REF) and/or division value N may be stored inmemory, for example control registers co-integrated with other lock-loopcircuitry and read on demand by circuitry of temperature monitor 202 orassociated processor circuitry.

Temperature monitor 202 may implement the functions T_(S)(S_(C)) orT_(S)(S_(C), F_(OUT)) by means of a look-up table, or by hard-wiredlogic to implement the required arithmetic, or by means of suitablesoftware code running on programmable processor circuitry. The look-uptable or local storage or coefficients necessary to implement thefunction T_(S)(S_(C)) or the software code may be stored non-transientlyin memory. This memory or the processor circuitry or other components oftemperature monitor 202 may be co-integrated with other components ofthe locked-loop circuitry.

In some instances the estimate of temperature T that is outputted may beused by some other circuit or device level control. In some instancesthe estimated temperature may be monitored by the temperature monitor202, e.g. to generate an alert if the temperature exceeds a definedoperating range or exhibits some defined characteristic such as a rapidincrease.

Additionally or alternatively the temperature monitor 202 may monitorthe value of control signal S_(C) against one or more thresholds,possibly with hysteresis, and generate an alert if said thresholds arecrossed. The thresholds may correspond to values of S_(C) correspondingto predefined values of temperature for defined operating ranges. Thethresholds may be selected or controlled based on the operating regimeof desired output frequency F_(OUT), reference frequency F_(REF) and/ordivision value N if appropriate. Thus the temperature monitor 202 couldgenerate an alert if the value of control signal S_(C) exceeds athreshold value indicating (for the relevant operating regime) anundesirably high temperature.

Whilst in some embodiments, where the output frequency F_(OUT) isvariable in use, the temperature monitor 202 may advantageously beprovided with an indication of the output frequency F_(OUT), in someembodiments the temperature monitor 202 may operate without knowledge ofthe actual output frequency F_(OUT) yet still be operable to monitortemperature changes.

For example consider that the output frequency F_(OUT) may be varied inuse (e.g. via variation of reference frequency F_(REF)) and the value ofthe desired output frequency F_(OUT) is not effectively available to thetemperature monitor 202. In steady state, when operating at a givenoutput frequency F_(OUT), the value of the control signal may not beexpected to change much in use, other than with temperature. In such acase the present value of the control signal S_(C) may not be able toprovide an estimate of the absolute temperature, since the value ofF_(OUT) and consequently the appropriate choice of function T_(S)(S_(C))would be unknown. However changes in the value of the control signalS_(C) may still usefully indicate dynamic temperature changes. Inparticular, if the value of the control signal S_(C) changes in arelatively continuous way in a direction that indicates an increase intemperature, then detection of such a change in the value of controlsignal S_(C) may indicate a continually increasing temperature andgenerate an alert, especially if the rate of change increases orpersists over a certain defined period of time. This could thus be usedas warning of a temperature change indicating possible thermal runaway.If the reference frequency F_(REF) or division ratio N were changed,this would likely lead to a more sudden change in the value of controlsignal S_(C) as the loop responds to the new regime and changes outputfrequency. The rate of change of the control signal S_(C) may thus jumphigh and then decrease. Changes in reference frequency may thus bedistinguished from temperature changes in such a way. In some instancehowever it may be beneficial to monitor the frequency error signal ΔFfor any step changes indicative of changes in reference frequencyF_(REF) rather than the filtered version providing the control signalS_(C) in order to detect such regime changes.

Some embodiments thus make use of a controlled oscillator 201 which isoperable with a transfer function K_(OSC) that varies with temperature.The transfer function K_(OSC) should advantageously vary significantlyacross the temperature range of interest so as to have a readilydetectable impact on the value of the control signal S_(C) required tomaintain the particular output frequency F_(OUT), i.e. the transferfunction K_(OSC) has a distinct temperature dependence. Advantageouslythe transfer function K_(OSC) may vary such that, the value of controlsignal S_(C) required to provide a given output frequency F_(OUT) variesin a monotonic way across the temperature range. Thus, for a givenoutput frequency F_(OUT), each value of the control signal S_(C) can beassociated with a unique temperature (or continuous temperature range).

FIGS. 3a and 3b illustrate one example of a suitable temperaturedependent transfer function. FIG. 3a illustrates how the outputfrequency F_(OUT) of the controlled oscillator 201 may vary withtemperature for a constant value of control signal S_(C). FIG. 3aillustrates a first plot 301 a of variation in output frequency F_(OUT)with temperature according to a first transfer function. It can be seenthat the output frequency F_(OUT) varies from a first frequency F₁ at atemperature T₁ to a second frequency F₂ at a second temperature T₂. Thevariation in output frequency F_(OUT) with temperature varies generallymonotonically across the temperature range so that each differenttemperature would lead to a different value of output frequency. Thevariation in output frequency with frequency is sufficient over thetemperature range of interest so that the variation in value of controlsignal S_(C) required to maintain a constant output frequency would bedetectable. FIG. 3b illustrates how the value of control signal S_(C)may vary with temperature to maintain a desired constant outputfrequency. FIG. 3b illustrates a plot 301 b of how the value of controlsignal may vary with temperature to compensate for the first transferfunction.

FIG. 3a illustrates the output frequency F_(OUT) may increase withtemperature for a fixed value of control signal S_(C), and FIG. 3billustrates that the value of control signal S_(C) required to maintaina desired output frequency may drop with increasing temperature. Howeverit should be understood that these are only examples and it would bepossible for example that the topology or bias of some controlledoscillator might lead its output frequency to decrease with increasingtemperature or that independently the polarity of the effect of acontrol signal might lead the value of control signal S_(C) required tomaintain a desired output frequency to increase with increasingtemperature. Likewise it should be noted that FIG. 3a illustrates agenerally linear variation across the temperature range. In someinstances a generally linear variation may occur in an implementationbut equally, in some instances, the temperature dependence of thetransfer function may vary in a significantly non-linear way over atleast part of the temperature range of interest.

It will be appreciated by one skilled in art that the temperaturedependence of the transfer functions illustrated by plots 301 a and 301b of FIGS. 3a and 3b may not be the temperature dependence that wouldnaturally occur in implementations of a controlled oscillator 201 of alocked-loop circuit such as an FLL. Typically the controlled oscillator201 would exhibit a generally flatter temperature response, e.g. atransfer function that exhibits less temperature variation. FIG. 3a alsoillustrates an example plot 302 a of how output frequency F_(OUT) mayvary with temperature for a second transfer function. The secondtransfer function may exhibit less temperature dependence than the firsttemperature characteristic. In this illustrative example there is someslight variation in output frequency F_(OUT) with temperature but theextent of any variation is more limited that for the first transferfunction, and there may be no overall trend. FIG. 3b illustrates, asplot 302 b, the value of control signal S_(C) required to maintain aconstant output frequency over the temperature range for such a secondtransfer function. The first transfer function may, for example, exhibita variation with temperature has a magnitude which is of the order of15% or greater, or 20% or greater, or 25% or greater than that for thesecond transfer function, where the second transfer function may be theconventional expected temperature response of a locked-loop circuitwhich is not used for temperature sensing.

The transfer function of the controlled oscillator 201 may be controlledin various ways, but in at least some examples a bias applied to theoscillator, for instance a bias current, may be controlled so that theoverall response of the oscillator has a desired temperature dependence.

FIG. 4 illustrates one example of a controlled oscillator 201. In thisexample the controlled oscillator 201 is controlled by a control signalS_(C) which may be a digital control word. Thus the controlledoscillator can be seen as type of numerically controlled oscillator. Thecontrolled oscillator 201 comprises a current controlled oscillator(ICO) 401, which may for instance be implemented by a suitable ringoscillator driven by a control current I_(C). As will be understood byone skilled in the art the ICO 401 may comprise a ring arrangement ofinverters, for example CMOS inverters. The control current I_(C) may beapplied to the supply rail of the inverters and thus controls thepropagation delay of the inverters.

A programmable current source 402, such as a current-outputdigital-to-analogue converter (IDAC), may comprise a number ofcontrollable current elements, with the number of elements contributingto the control current I_(C) being selected by the value of the digitalword of the control signal S_(C). The currents deliverable by thecontrollable current elements may be equally or binary-weighted and bescaled with respect to a supplied bias current I_(BIAS) which determinesthe current contribution to I_(C) of each current element when selected.Thus the output current I_(C) for each value of control signal S_(C) maybe proportional to the value of I_(BIAS). Any variation with temperatureof I_(BIAS) will also lead to a proportionate variation in I_(c).

In practice the ICO 401 will have a particular temperature dependencebetween the control current I_(C) and frequency F_(OUT) of the outputsignal S_(OUT), e.g. due to the relationship between current andpropagation delay of the inventors and how it varies with temperature.For conventional biasing of controlled oscillators the bias currentI_(BIAS) may thus be designed to have a temperature response that meansthat the overall temperature response of the controlled oscillator issubstantially flat. A bias generation circuit 403 may thus have at leastone circuit branch having a current source 404 for generating a currentI_(PTAT) which is proportional to absolute temperature, i.e. increaseswith increasing temperature, and/or at least one circuit branch having acurrent source 404 for generating a current I_(CTAT) which iscomplementary to absolute temperature, i.e. which decrease with absolutetemperature. The currents I_(PTAT) and I_(CTAT) may be generated in aratio of α:β so as to define an overall bias current I_(BIAS) which hasthe desired variation with temperature, which may conventionally bearranged so as to effectively provide the control current I_(C) with atemperature variation (for a defined value of control word of thecontrol signal S_(C)) that substantially compensates for the temperaturevariation of the ICO 401, e.g. to provide a temperature response asillustrated by plots 302 a and 302 b in FIGS. 3a and 3 b.

In embodiments of the present disclosure however the bias circuit may beoperable to generate a defined bias current I_(BIAS) that provides anoverall transfer function with a significant temperature dependence,such that over a temperature range of interest a variation intemperature would result in a measurable change in the value of thecontrol word of the control signal S_(C), such that the value of controlsignal S_(C) required to maintain the required output frequency F_(OUT)can be used as an indication of temperature. Thus the bias generationcircuit 403 may be operable to generate a bias current I_(BIAS) so as toprovide an overall temperature response as illustrated by plots 301 aand 301 b in FIGS. 3a and 3 b.

In some embodiments the controlled oscillator 201 may be configured soas to have a first transfer function K_(OSC) that varies withtemperature in a defined way, such that the value of the control signalS_(C) can be used as an indication of temperature, and the controlledoscillator 201 may, in use, always be operated to provide the firsttransfer function, i.e. a temperature dependent transfer function suchas illustrated by plots 301 a and 301 b. In use this means therelationship between the value of the control signal S_(C) and theoutput frequency F_(OUT) will vary with temperature. However theoperation of the FLL feedback loop will maintain the output frequencyF_(OUT) at the desired value of N·F_(REF) irrespective of temperaturevariations. Thus it is possible to operate the locked-loop circuit 200to provide a desired output signal and, at the same time, monitortemperature, e.g. to detect onset or likelihood of thermal runaway.

In at least some embodiments however the controlled oscillator 201 maybe operable in at least first and second modes, where the temperaturedependence of the controlled oscillator 201 is different in the firstand second modes. In the first mode, which may be a temperature sensingmode, the controlled oscillator 201 may have a first transfer functionwhich is temperature dependent transfer function K_(OSC1) that has adesired variation with temperature such that the value of the controlsignal S_(C) can be used to estimate temperature as described above. Inthe second mode, which may be a mode that is used when temperaturesensing capability is not required, the controlled oscillator 201 may bearranged to have a second transfer function K_(OSC2) that is largelytemperature independent or which exhibits a lesser degree of temperaturevariation compared to the first operating mode. The first transferfunction may, for example, exhibit a variation with temperature has amagnitude which is of the order of 15% or greater, or 20% or greater, or25% or greater than that for the second transfer function. Thus thevariation in frequency over the defined temperature range may be of theorder of at least 20% greater for a controlled oscillator exhibiting thefirst transfer function rather than the second transfer function. Forexample the first transfer function K_(OSC1) could provide a temperatureresponse such as illustrated by plots 301 a and 301 b in FIGS. 3a and 3b, whereas the second transfer function K_(OSC2) may provide atemperature response along the lines as illustrated by plots 302 a and302 b.

Referring back to FIG. 2, the controlled oscillator 201 may thus beresponsive to a mode control signal, MODE, to enable or disabletemperature sensing and to control the mode of operation of thecontrolled oscillator 201 and the temperature dependence of the transferfunction accordingly. The mode control signal MODE may be controlled bya signal received from some other component, such a system controller orapplications processor of the like. In some instances the mode controlsignal MODE may also enable or disable the temperature monitor asappropriate.

The mode of operation of the controlled oscillator 201 could, in someembodiments, be implemented by selectively controlling the biasingapplied. For instance as illustrated in FIG. 4 the mode control signalMODE could be used to control current sources 404 and/or 405 to vary thecontribution of at least one of the I_(PTAT) and I_(CTAT) currents, e.g.to vary the ratio α:β.

Embodiments thus enable the use of a locked-loop circuit 200, such as anFLL, to be used both to operate as locked circuit and also to provide atemperature sensing functionality. Locked-loop circuits are commonlyused for clock synthesis, e.g. to generate a clock signal with a desiredfrequency. In order to allow for different components to the clocked atdifferent rates in use it may be common for at least some applicationsto provide a plurality of locked-loop circuits on an integrated circuit.Embodiments thus may provide temperature sensing functionality usingcomponents that would in any case be present as part of the integratedcircuit.

Locked-loop circuits according to embodiments can be used to providetemperature sensing in use in a number of ways. If a particularlocked-loop circuit 200 is required to operate to generate a desiredoscillation signal, e.g. to generate a clock signal at a particularfrequency F_(OUT), and it is also wished to also provide temperaturemonitoring, the controlled oscillator 201 of the locked-loop circuit 200may be operated in a first mode to provide a first, temperaturedependent, transfer function and the temperature monitor 202 enabled tomonitor temperature. If however there are multiple locked-loop circuitsand not all the locked-loop circuits are required to generate a clocksignal, then one or more locked-loop circuit may be operated to generatethe clock signal(s) without necessarily providing temperature sensing.In which case, in some implementations, a locked-loop circuit which isoperating just to generate the respective required output frequencycould be operated with the controlled oscillator operating in a secondmode to provide a second transfer function that may exhibit lesstemperature variation than in the first mode. At least one of thelocked-loop circuits which is not required for clock signal generationcould be operated purely to provide temperature monitoring, in whichcase the relevant controlled oscillator 201 may be operated in the firstmode.

A locked-loop circuit 201 which is being operated purely to providetemperature sensing may be operated as a locked-loop, i.e. with thefeedback loop active so that the value of the control signal S_(C) iscontrolled to maintain a desired output frequency F_(OUT), in which casethe temperature monitor 202 may operate as describe previously.

In some embodiments however the locked-loop circuit may also be operablein an open-loop mode, so as to generate a certain output frequencyF_(OUT) based on a selected fixed value of control signal S_(CF) whichis not controlled by the feedback loop.

Such an open loop mode may, for instance, be used on start-up so thatthe controlled oscillator 201 is initially controlled based on aselected fixed value of control signal S_(CF), which may be an estimateof the likely required value of the control signal S_(C) when operatingin closed-loop mode. The circuit may operate in the open-loop mode for aperiod before switching to the closed-loop mode which may reduce thetime required to lock onto the required output frequency F_(OUT).

Whether or not such an open-loop mode may be used for start-up of thelocked loop circuit, in some embodiments an open loop mode may beimplemented to provide temperature monitoring without requiring thewhole locked-loop circuit to be active. In an open-loop mode a fixedvalue of control signal S_(CF) may be supplied to the controlledoscillator which is operated to have a first transfer function thatvaries with temperature as described above. In this instance the outputfrequency is monitored to determine an indication of the outputfrequency F_(OUT) that is generated. In this instance the control signalvalue is fixed S_(CF) and the output frequency is monitored, e.g. as towhether the frequency is above or below a frequency threshold thatcorresponds to a defined temperature threshold.

As illustrated in FIG. 5 in some embodiments therefore there may be aswitch arrangement 501 such as a multiplexor or the like that canreceive a define fixed control signal value, S_(CF), e.g. a definedcontrol word and also a control signal S_(C) from controller 102 andsupply a selected one of these control signals, S_(CF) or S_(C) to thecontrolled oscillator based on a select signal SEL, which in someinstances could the same signal as, or derived from, the mode controlsignal MODE. In such an embodiment the temperature monitor 202 mayreceive a version of the output signal S_(OUT) in order to determine anindication of the output frequency F_(OUT). It will be appreciated thatin such an embodiment the locked-loop circuit 200 will still be operableas a locked-loop circuit, i.e. in closed loop mode, when required.

When operating in the open-loop mode, i.e. with S_(CF) applied as thecontrol signal for to the controlled oscillator, an indication of theoutput frequency F_(OUT) may conveniently be obtained by counting thepulses of S_(OUT) by a counter reset and sampled on an edge of a slowerclock, for example S_(REF) if available or some divided-down versionthereof or some other known clock of known frequency available to thecircuit. This will thus provide an indication of the ratio of the outputfrequency F_(OUT) to the slower clock used. This value may then be usedsimilarly to the control signal S_(C) used in other modes: for exampleit may be compared to a threshold value or may be used as input to somefunction which will determine an estimate of temperature based on theindication of frequency. The threshold and/or the transfer function maybe determined on the basis of prior simulation or measurements.

In all the embodiments above, it is assumed that a transfer functionfrom control signal value S_(C) to an estimate of temperature, or acontrol signal threshold value S_(C) corresponding to a definedtemperature (or an analogous function in terms of output frequency foran open-loop mode) is available to the circuit. Coefficients for suchfunctions or threshold values may be determined form simulation and/orfrom measurement of representative samples of circuitry.

However in practice there may be substantial sample-to-sample variationin signal value S_(C) due to manufacturing tolerances. There even besome variation, albeit generally much smaller, between circuitsintegrated on the same semiconductor die. In some embodiments a widevariation in say the temperature at which the occurrence of anundesirable temperature is to be flagged may be acceptable, for instancein a crude thermal runaway detector. However more accuracy may berequired in some embodiments. For example a more precise measurement oftemperature might be necessary to allow say a system clock frequency tobe tightly managed by a control loop to be at the maximum allowableconsidering some temperature limit in order to maximise computing power.In such embodiments, the variation may require to be reduced.

The variations may be considered in two classes. There may besample-to-sample variations in the current-to-frequency transferfunction K_(C) between the controlled current I_(C) and the outputfrequency F_(OUT) of the controlled oscillator. There may also belargely uncorrelated variations in the control current I_(C) for a givencode S_(C) due for example to variability in the value of I_(BIAS).

The variations may be reduced by measuring each manufactured sample andadjusting or trimming each sample accordingly. Trimming may involvestoring some value as a digital word in some non-volatile storage wherethe digital word programs the value of some element such as a resistorin the circuitry. The word may be chosen, possibly in an iterativeprocess, so as to adjust say the value of S_(C) that will produce agiven output frequency at room temperature to a desired nominal value ofS_(C).

Some embodiments may employ a relatively simple trimming procedure; someembodiments may be required to provide more accuracy and thus a morecomplicated trimming procedure may be necessary.

In some embodiments it may be sufficient to trim the value of I_(BIAS),so that the residual variation in S_(C) is due only to variation of theoscillator current-to-frequency transfer function K. In someembodiments, to achieve more accuracy, it may be necessary to also trimthe transfer function of the oscillator, for instance by programmingsome secondary control voltage applied to say the back-bias voltage orgate bias voltage of some transistors in the circuit, or by programmingsome digitally-controllable load capacitance in one or more stages ofthe oscillator. In some embodiments it may be necessary to store anindication of the measured value of S_(C) and to use this as an extrainput variable to the single threshold value or to the transfer functionequations used to derive temperature measure or to select the mostappropriate one of a family of transfer function equations. There maystill be some residual variation in the dependence of S_(C) withtemperature, but this may be only a relatively small effect.

The embodiments discussed above have been described with respect to anFLL arrangement. However temperature monitoring could be implemented ina similar way for other locked-loop circuits such as a PLL for example.A PLL would operate similarly to the FLL, but with a Phase detector orPhase-Frequency detector instead of the Frequency detector. Temperaturemonitoring could also be implemented using a DLL.

FIG. 6 illustrates an example of a DLL arrangement 600 operable as atemperature sensor. The reference signal S_(REF) is input to a delayline 601.

FIG. 7 illustrates an example of a suitable delay line 601 which maycomprise a series connection of a plurality of delay elements 701 thedelay of each of which is controlled by a control signal S_(C) as wouldbe understood by one skilled in the art. The output from one of thedelay elements 701 may be tapped by delay line output block 702 to forman output signal S_(OUT). By selecting the output of the relevant delayelement 701 the output signal may have a desired delay or phase shift tothe input reference signal S_(REF). In some embodiments there may bemore than one output signal, for instance a set of clock signals of thesame frequency but different delays or phases, generated by output block702 tapping outputs of one or more other delay elements of the delayline.

In some embodiments at least one output signal may be generated bylogically combining, in output block 702, signals from outputs of chosendelay elements 701 of the delay line 601 as will be understood by thoseskilled in the art. This may provide an output signal at a differentfrequency to the reference signal S_(REF), and/or allow output signalsof different duty cycles. In some embodiments waveforms at outputs ofchosen delay elements of the delay line may be combined in an analoguefashion to obtain outputs with intermediate values of delay, using forexample known phase interpolation techniques.

A feedback signal S_(FB) (possibly one of the above output signals) mayalso be obtained by tapping an output from one of the delay elements,possibly the final delay element as illustrated. FIG. 7 illustrated thefeedback signal S_(FB) derived directly from the end of the delay linebut it will be understood the feedback signal could be output via outputblock 702. In some embodiments, a signal obtained from intermediatenodes within the delay line 601 by one or more of the techniques of theprevious paragraph may be employed as the feedback signal S_(FB).

Returning to FIG. 6, the feedback signal S_(FB) is fed back tocontroller 602, which also receives the reference signal S_(REF). Errorblock 603 determines the extent of any phase error between the feedbacksignal S_(FB) and a desired phasing with the reference signal S_(REF).The phase error ΔP may be filtered by loop filter 604 to form a controlsignal S_(C) for controlling the delay line 601 by operativelycontrolling the delays of the delay elements 701 that make up the delayline 601. In other words, the purpose of the control signal S_(C) is tomaintain a certain timing relationship between the feedback signal andthe reference signal.

The delay line 601 will have a transfer characteristic K_(DEL) betweenthe value of the control signal S_(C) and the delay provided between theinput reference signal S_(REF) and the feedback signal S_(FB) by thedelay line 601. This delay line 601 transfer characteristic K_(DEL) mayinherently vary with temperature in a known way as the delay elementsmay have a temperature dependence as would be understood by one skilledin the art. In a similar manner as discussed above with respect to thecontrolled oscillator, the delay line 601 could be implemented, i.e.engineered, to have a transfer characteristic K_(DEL) that has adistinct, i.e. usefully large, variation with temperature and/or thedelay line 601 may be operable in different modes of operation whereinin at least one mode of operation the transfer characteristic K_(DEL)has a distinct variation with temperature. At least some of thetemperature variation could be derived from a bias circuit similar tothat discussed above. In use, the value of the control signal S_(C)required to maintain feedback signal S_(FB) such as to have a desiredphase relationship, i.e. such as to have a certain timing relationship,with the reference signal S_(REF) will thus vary with temperature andconsequently the value of the control signal S_(C) may be monitored by atemperature monitor 202 in a similar way as in the embodiments discussedabove.

In all the locked-loop circuits discussed, the locked-loop circuit isoperable to determine a timing difference, e.g. any difference infrequency and/or phase, between a reference signal and an a feedbacksignal and determine a control signal for a signal timing module basedon the determined difference. In other words, in all the locked-loopcircuits discussed, the locked-loop circuit is operable to determine atiming relationship, e.g. a relationship in frequency and/or phase,between a reference signal and a feedback signal and determine a controlsignal for controlling a signal timing module based on the determineddifference. The signal timing module for a PLL or FLL may be acontrolled oscillator which generates an oscillation signal to be outputof the PLL or FLL, whereas for a DLL the signal timing module may be adelay line that receives the reference signal and applies a controlleddelay. In embodiments of the disclosure the transfer characteristic ofthe signal timing module varies with temperature so that the value ofthe control signal required to maintain the output signal to have thedesired, i.e. a certain, timing relationship to the reference signalvaries with temperature. The control signal can thus be monitored toprovide temperature sensing.

The skilled person will thus recognise that some aspects of the abovedescribed apparatus and methods may be embodied as processor controlcode, for example on a non-volatile carrier medium such as a disk, CD-or DVD-ROM, programmed memory such as read only memory (Firmware), or ona data carrier such as an optical or electrical signal carrier. For manyapplications, embodiments of the present invention will be implementedon a DSP (Digital Signal Processor), ASIC (Application SpecificIntegrated Circuit) or FPGA (Field Programmable Gate Array). Thus thecode may comprise conventional program code or microcode or, forexample, code for setting up controlling an ASIC or FPGA. The code mayalso comprise code for dynamically configuring re-configurable apparatussuch as re-programmable logic gate arrays. Similarly, the code maycomprise code for a hardware description language such as Verilog™ orVHDL. As the skilled person will appreciate, the code may be distributedbetween a plurality of coupled components in communications with oneanother. Where appropriate, the embodiments may also be implementedusing code running on a field-(re)programmable analogue array or similardevice in order to configure analogue hardware.

Embodiment of the present disclosure may be implemented as an integratedcircuit. Embodiments of the present disclosure may be implemented in anelectronic device. The electronic device may be at least one of: aportable device; a battery powered device; a communication device; amobile or cellular telephone or a smartphone; a computing device; alaptop, notebook or tablet computer; a gaming device; a wearable device;a voice controlled device.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. The word “comprising” does not excludethe presence of elements or steps other than those listed in the claim,“a” or “an” does not exclude a plurality, and a single feature or otherunit may fulfil the functions of several units recited in the claims.Any reference numerals or labels in the claims shall not be construed soas to limit their scope. Terms such as amplify or gain include possibleapplying a scaling factor or less than unity to a signal.

The invention claimed is:
 1. A locked-loop circuit comprising: acontrolled oscillator operable to generate an output signal at an outputfrequency based on the value of a control signal; and a controlleroperable to compare a feedback signal derived from the output signal toa reference signal received at a reference signal input to generate saidcontrol signal; wherein the controlled oscillator is operable in a firstmode in which a value of the control signal required to maintain acertain output frequency changes with temperature; and the circuitcomprises a temperature monitor for monitoring a temperature based onthe value of the control signal; wherein, in the first mode thecontrolled oscillator has a first transfer function between the outputfrequency and the value of the control signal, and wherein thecontrolled oscillator is further operable in a second mode with a secondtransfer function between the output frequency and the value of thecontrol signal that exhibits a lesser temperature dependence than thefirst transfer function.
 2. A locked-loop circuit as claimed in claim 1,wherein the temperature monitor is configured to monitor the value ofthe control signal for changes that indicate a continuing increase intemperature.
 3. A locked-loop circuit as claimed in claim 1, wherein thecontrol signal comprises a digital signal having a value defined by adigital value.
 4. A locked-loop circuit as claimed in claim 1, whereinthe second transfer function exhibits substantially no variation withtemperature over a defined temperature range.
 5. A locked-loop circuitas claimed in claim 1, wherein the first transfer function exhibits avariation with temperature over a defined temperature range which has amagnitude which is 20% or greater than that for the second transferfunction.
 6. A locked-loop circuit as claimed in claim 1, wherein thecircuit is configured to be responsive to a control signal to enable ordisable temperature sensing, with the controlled oscillator operating inthe first mode when temperature sensing is enabled and in the secondmode when temperature sensing is not enabled.
 7. A locked-loop circuitas claimed in claim 1, wherein the locked-loop circuit is afrequency-locked-loop circuit.
 8. A locked-loop circuit as claimed inclaim 1, wherein the locked-loop circuit is a phase-locked-loop circuit.9. A locked-loop circuit as claimed in claim 1, implemented as anintegrated circuit.
 10. A locked-loop circuit as claimed in claim 1,wherein the temperature monitor is configured to monitor the value ofthe control signal against one or more thresholds and to generate analert if said thresholds are crossed.
 11. A locked-loop circuit asclaimed in claim 10, wherein the circuit is operable such that theoutput frequency is variable in use and wherein said one or morethresholds are selected based on an indication of the output frequency.12. A locked-loop circuit as claimed in claim 1, wherein the temperaturemonitor is configured to determine an estimate of a present temperaturebased on the value of the control signal.
 13. A locked-loop circuit asclaimed in claim 12, wherein the circuit is configured such that theoutput frequency is variable in use and wherein the temperature monitoris configured to determine the estimate of the present temperature basedon the value of the control signal and on an indication of the outputfrequency.
 14. A locked-loop circuit as claimed in claim 1, furthercomprising a switching arrangement in a signal path between thecontroller and the controlled oscillator, wherein the circuit isoperable in a closed-loop mode in which the switching arrangement isconfigured to supply the control signal to the controlled oscillatorfrom the controller and is further operable in an open-loop mode inwhich the switching arrangement is configured to supply a definedcontrol value to the controlled oscillator as the control signal.
 15. Alocked-loop circuit as claimed in claim 14, wherein, in the open-loopmode the temperature monitor is configured to receive a version of theoutput signal generated in response to the defined control value and tomonitor the temperature by monitoring the frequency of the outputsignal.
 16. A locked-loop circuit as claimed in claim 1, wherein thecontrolled oscillator comprises a bias circuit for generating a bias forthe controlled oscillator, wherein the bias circuit is configured togenerate the bias having a controlled temperature dependence so as toprovide a first transfer function for the controlled oscillator betweenthe output frequency and the value of the control signal, wherein thefirst transfer function has a distinct temperature dependence.
 17. Alocked-loop circuit as claimed in claim 16, wherein the controlledoscillator comprises a current controlled oscillator and a programmablecurrent source controlled by the control signal wherein the bias circuitsupplies the bias to the programmable current source.
 18. A locked-loopcircuit as claimed in claim 16, wherein the bias circuit is configurableso as to provide a bias with a first temperature dependence in the firstmode and a second temperature dependence in the second mode.
 19. Alocked-loop circuit as claimed in claim 1, wherein the circuit isoperable such that the output frequency is variable in use and thetemperature monitor is configured to monitor the temperature based onthe value of the control signal and an indication of the outputfrequency.
 20. A locked-loop circuit as claimed in claim 19, wherein thetemperature monitor is configured to receive a version of the outputsignal and determine said indication of the output frequency.
 21. Alocked-loop circuit as claimed in claim 19, wherein the circuitcomprises a frequency divider configured to apply a frequency divisionto a version of the output signal based on a controllably variabledivision value to provide said feedback signal and wherein thetemperature monitor is provided with said division value as at leastpart of the indication of the output frequency.
 22. A locked-loopcircuit as claimed in claim 19, wherein the temperature monitor isconfigured to receive an indication of a reference frequency as at leastpart of said indication of the output frequency.
 23. An electronicdevice comprising a locked-loop circuit as claimed in claim
 1. 24. Anelectronic device as claimed in claim 23, wherein the device is at leastone of: a portable device; a battery powered device; a communicationsdevice; a mobile or cellular telephone; a smartphone; a computingdevice; a notebook, laptop or tablet computing device; a wearabledevice; a smartwatch; a voice-controlled device; and a gaming device.25. A method of temperature monitoring comprising: operating alocked-loop circuit in a first mode or a second mode to compare afeedback signal derived from an output signal to a reference signalreceived at a reference signal input to generate a control signal; andmonitoring a temperature based on the value of the control signal;wherein, in the first mode the locked-loop circuit has a first transferfunction between a frequency of the output signal and the value of thecontrol signal, and wherein in the second mode the locked-loop circuithas a second transfer function between the output of the output signaland the value of the control signal that exhibits a lesser temperaturedependence than the first transfer function.
 26. A locked-loop circuitcomprising: a controlled oscillator operable in a first mode with afirst transfer function that varies with temperature such that a valueof a control signal required to maintain a certain output frequencychanges with temperature across a defined temperature range and furtheroperable in a second mode with a second transfer function that exhibitsa lesser temperature dependence than the first transfer function; and atemperature monitor for monitoring a temperature based on therelationship between the value of the control signal and the outputfrequency when the controlled oscillator is operating in said firstmode.
 27. A locked-loop circuit comprising: a controlled delay lineoperable to receive an input signal, apply a controlled delay to theinput signal and output at least a first signal based on the delayedinput signal wherein the controlled delay applied is based on the valueof a control signal; and a controller operable to compare a feedbacksignal derived from the first signal to a reference signal received at areference signal input to generate said control signal; wherein thecontrolled delay line is operable such that a value of the controlsignal required to maintain a certain delay changes with temperature;and the circuit comprises a temperature monitor for monitoring atemperature based on the value of the control signal.
 28. A locked-loopcircuit comprising: a reference input for receiving a reference signal;a controlled delay line operable to apply delays to the reference signalto provide one or more delayed signals and to output a feedback signalbased on one or more of the delayed signals, wherein the delays arecontrolled dependent on a value of a control signal, wherein thecontrolled delay line is operable in a mode in which the delays are alsodependent on a circuit temperature; and a controller operable togenerate said control signal at a control signal value required tomaintain a timing relationship between the feedback signal and thereference signal at the circuit temperature; wherein the circuitcomprises a temperature monitor for monitoring a temperature based onthe value of the control signal.